The invention relates to the field of alkali metal-based, preferably lithium-based, energy storage systems, more particularly lithium-ion batteries with stability to high voltages.
Lithium-ion batteries currently represent the leading technology in the field of rechargeable batteries. Lithium-ion batteries are currently used particularly in portable electronics, and lithium-ion batteries for larger systems such as electric vehicles are in development. A conductive salt currently used in commercially available lithium-ion batteries is lithium hexafluorophosphate (LiPF6). In this case the conductive salt is in solution in a nonaqueous solvent, usually a mixture of carbonates.
A disadvantage of the conventional electrolytes based on lithium hexafluorophosphate in carbonates lies in particular in the low oxidative stability of 4.5 V against Li/Li+. The electrolyte is stable only up to this voltage, while outside of this range there is oxidative decomposition of the electrolyte and, in association therewith, dissolution of the cathode material. For lithium-ion batteries with high energy density or high power density as well, which preferably use lithium nickel manganese cobalt mixed oxides (NMC), or lithium nickel manganese oxide (LNMO) as active material for the cathodic electrode, electrolyte decomposition and cathode-material dissolution occur at end-of-charge voltages above 4.4 V or 4.7 V, respectively. The consequence is a low cycling stability and hence short lifetime of the battery.
In order to improve the lifetime at elevated temperature, EP 1 056 143, for example, proposes using composite cathodes produced from a lithium manganese oxide suspension combined with extraneous-metal compounds, such as Mg(NO3)2. The suspension is applied to the collector and cured above the decomposition temperature of the metal compound at 350° C., for example. It is known, from Kang Y. C. et al., Journal of Ceramic Processing Research, vol. 9, No. 2, pp. 140-145 (2008), for example, that Mg(NO3)2 exhibits an endothermic peak in the DSC at 330° C., which is indicative of decomposition or conversion to MgO.
It is therefore an object of the invention to provide means suitable for improving the cycling stability or lifetime of an alkali metal-ion battery, more particularly lithium-ion battery, with elevated end-of-charge potentials.
This and other objects of the invention are achieved by means of an alkali metal-based, more particularly lithium-based, energy storage system having at least one composite electrode, more particularly a composite cathode, having an active material, and an electrolyte having an alkali metal salt, more particularly a lithium salt, dissolved in solution with an aprotic organic solvent, an ionic liquid and/or a polymer matrix. The electrolyte also includes an additive selected from a cation or a compound of a metal selected from Mg, Al, Cu and/or Cr. In one aspect, the active material of the cathode bears a metal selected from Mg, Al, Cu and/or Cr applied by sputtering. In another aspect, the active material of the composite electrode, more particularly composite cathode, is partially replaced by a metal selected from Mg, Al, Cu and/or Cr in the form of a metal powder or metal salt.
Surprisingly it has been found that the addition of a metal cation selected from Mg, Al, Cu and/or Cr and/or the sputter deposition of Mg, Al, Cu or Cr onto the cathode, and/or the partial replacement of the active material of the composite electrode, more particularly composite cathode, by a metal selected from Mg, Al, Cu and/or Cr in the form of a metal powder or a metal salt, is able to provide a higher cycling stability and a longer lifetime of an alkali metal battery, more particularly of a lithium-ion battery. This makes it possible for the battery to be charged up to higher end-of-charge potentials, without destruction of the individual cell components containing a cathode, an anode, and an electrolyte. As a result, the stability window and hence operational window of the battery is enlarged. A particular advantage in this case is that the higher potentials that are enabled result in a higher energy density of the battery. Moreover, the self-discharge, which constitutes one of the greatest problems when using lithium nickel manganese oxide (LNMO) as cathode active material, is considerably reduced.
The effect of adding Mg, Al, Cu and/or Cr may be achieved firstly by the addition in the form of electrolyte additive, secondly by application, more particularly sputter deposition, of the metal onto the cathode active material, and lastly by partial replacement of the active material of the composite electrode, more particularly composite cathode, by a metal selected from Mg, Al, Cu and/or Cr in the form of a metal powder or a metal salt, preferably Mg, for example, MgSO4, during the production of the composite electrode, more particularly a composite cathode, from a current collector and an active-material suspension.
One aspect of the invention relates to an electrolyte for an alkali metal-based, more particularly lithium-based, energy storage system, having at least one alkali metal salt, more particularly a lithium salt, dissolved in solution with an aprotic organic solvent, an ionic liquid and/or a polymer matrix, and at least one additive selected from a cation or a compound of a metal selected from Mg, Al, Cu and/or Cr. A cell containing an electrolyte having an additive of the invention may advantageously exhibit lower impedance than a corresponding cell without the additive. It has been found, moreover, that the capacity decrease at elevated charge and discharge rates was reduced. Without being bound to any particular theory, it is assumed that the addition of the additive leads to the formation of a passivating layer on the cathode. This layer is able to protect electrode and electrolyte from decomposition. It is further assumed that the metals of the invention in the form of cations are irreversibly inserted into the respective cathode materials and protect the material from degradation.
Magnesium, aluminum, copper, and chromium may be added in the form of salt like, inorganic or organic compounds to the electrolyte. The cation is present preferably with a counterion, which also serves as anion of a conductive salt. In preferred embodiments, the metal selected from Mg, Al, Cu and/or Cr is present as a cation of a magnesium, aluminum, copper and/or chromium salt in conjunction with an anion selected from AsF6−, ClO4−, SbFb−, PtCl6−, AlCl4−, GaCl4−, SCN−, AlO4−, CF3CF2SO3−, (CF3)SO3−, C(SO2CF3)3−, PF6−, PF3(CF3)3− (FAP), PF4(C2O4)−, BF4−, B(C2O4)2−, BF2(C2O4)−, B(C2O4)(C3O4)−, (C2F5BF3)− (FAB), B12F122−, N(SO2CF3)2−, N(FSO2)2− and/or N(SO2C2F5)2−. More particularly, Mg, Al, Cu, and Cr may be used as a mixture of a lithium-based conductive salt with a magnesium-, aluminum-, copper- or chromium-based conductive salt. Advantageously, therefore, the high-voltage stability of alkali metal-based, more particularly lithium-based, secondary batteries, or accumulators, can be improved, while likewise the conductivity of the electrolyte is supported. The anion is preferably selected from bis(trifluoromethanesulfonyl)imide (TFSI), bis(fluorosulfonyl)imide (FSI), hexafluorophosphate, hexafluoroarsenate, perchlorate, tetrafluoroborate, difluoromono(oxalate)borate, bioxalatoborate and/or trifluoromethanesulfonate (Tf). Mg, Al, Cu, and Cr may be present more particularly in the form of a sulfonylimide salt or sulfonate salt.
As used herein, the term “additive” refers to a material or compound which is added to the electrolyte in just a small amount, more particularly in an amount of not greater than 10 wt %, based on the total electrolyte weight. In preferred embodiments, the electrolyte includes the magnesium, aluminum, copper and/or chromium salt in the range from ≥0.1 ppm to ≤10 wt %, preferably in the range from ≥0.01 wt % to ≤5 wt %, more preferably in the range from ≥0.1 wt % to ≤2 wt %, based on a total electrolyte weight of 100 wt %. For example, the electrolyte may contain 0.7 wt % or 1 wt % of the magnesium, aluminum, copper and/or chromium salt. As defined herein, 1 ppm (parts per million) stands for 0.0001 wt %. It has been observed that even amounts of 0.1 wt % or 0.7 wt % of the metal salt were sufficient for a decrease in capacity loss with increasing number of cycles (capacity fading). Good effects were achieved in particular in the range from between ≥0.1 wt % to ≤1 wt %.
The electrolyte may in particular contain a magnesium salt or aluminum salts as an additive. In preferred embodiments, the electrolyte contains magnesium(II) bis (trifluoromethanesulfonyl)imide (mgTFSI2) or aluminum(III) trifluoromethanesulfonate (AlTf3). It has been observed that these salts as electrolyte additive gave particularly good improvements in the cycling stability.
In another aspect, the electrolyte includes an alkali metal salt, more particularly a lithium salt, dissolved in solution with an aprotic organic solvent, an ionic liquid and/or a polymer matrix.
Examples of the organic solvent may be selected from ethylene carbonate, propylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, acetonitrile, glutaronitrile, adiponitrile, pimelonitrile, gamma-butyrolactone, gamma-valerolactone, dimethoxyethane, 1,3-dioxalane, methyl acetate, ethyl methanesulfonate, dimethyl methyl phosphonate, and/or a mixture thereof. Suitable organic solvents are from cyclic carbonates such as ethylene carbonate and propylene carbonate and/or linear carbonates such as diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate. Preferably, the organic solvent is selected from ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate and/or mixtures thereof. Also preferred are binary mixtures of carbonates, more particularly of ethylene carbonate with a further carbonate such as ethyl methyl carbonate, diethyl carbonate, or dimethyl carbonate.
Further preferred solvents are ionic liquids. These liquids have high thermal and electrochemical stability and also good ionic conductivity. Preferred ionic liquids has a cation selected from 1-ethyl-3-methylimidazolium (EMI+), 1,2-dimethyl-3-propylimidazolium (DMPI+), 1,2-diethyl-3,5-dimethylimidazolium (DEDMI+), trimethyl-n-hexylammonium (TMHA+), N-alkyl-N-methylpyrrolidinium (PYRIR+), N-alkyl-N-methylpiperidinium (PIPIR+) and/or N-alkyl-N-methylmorpholinium (MORPIR+), and an anion selected from bis(trifluoromethanesulfonyl)imide (TFSI), bis(pentafluoroethanesulfonyl)imide (BETI−), bis(fluorosulfonyl)imide (FSI−), 2,2,2-trifluoro-N-(trifluoromethanesulfonyl)acetamide (TSAC−), tetrafluoroborate (BF4−), pentafluoroethanetrifluoroborates (C2F5BF3−), hexafluorophosphate (PF6−) and/or tris(pentafluoroethane)trifluorophosphate ((C2F5)3PF3−). Preferred N-alkyl-N-methylpyrrolidinium (PYRIR+) cations are selected from N-butyl-N-methylpyrrolidinium (PYR14+) and/or N-methyl-N-propylpyrrolidinium (PYR13+). Preferred ionic liquids are selected from N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14TFSI) and/or N-methyl-N-propylpyrrolidinium bis (trifluoromethanesulfonyl)imide (PYR13TFSI).
Further suitable are polymer electrolytes, in which case the polymer electrolyte may take the form of a gel polymer electrolyte or solid polymer electrolyte. Solid polyelectrolytes allow a solvent-free construction which is easy to produce and diverse in its form. Furthermore, the energy density can be increased, since only a thin polymer film is required between the electrodes. Solid electrolytes in general are chemically and electrochemically stable toward electrode materials, and also do not escape from the cell.
Gel polymer electrolytes usually include an aprotic solvent and a polymer matrix. Examples of preferred polymers for solid polymer electrolytes and gel polymer electrolytes include homo- or copolymers of polyethylene oxide (PEO), polypropylene oxide (PPO), polyvinylidene fluoride (PVdF), polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP), polyacrylonitrile (PAN), polymethyl methacrylate (PMMA), polyethyl methacrylate (PEMA), polyvinyl acetate (PVAc), polyvinyl chloride (PVC), polyphosphazene, polysiloxanes, polyvinyl alcohol (PVA) and/or homo- and (block) copolymers having functional side chains selected from ethylene oxide, propylene oxide, acrylonitrile and/or siloxanes.
Examples of lithium salts with conductive salt suitability include LiAsF6, LiClO4, LiSbF6, LiPtCl6, LiAlCl4, LiGaCl4, LiSCN, LiAlO4, LiCF3CF2SO3 Li(CF3)SO3, (LiTf), LiC(SO2CF3)3, phosphate-based lithium salts such as LiPF6, LiPF3(CF3)3 (LiFAP), and LiPF4(C2O4) (LiTFOB), borate-based lithium salts such as LiBF4, LiB(C2O4)2 (LiBOB), LiBF2(C2O4) (LiDFOB), LiB(C2O4)(C3O4) (LiMOB), Li(C2F5BF3) (LiFAB), and L12B12F12 (LiDFB) and/or lithium salts of sulfonylimides, preferably LiN(SO2CF3)2 (LiTFSI) and/or LiN(SO2C2F5)2 (LiBETI). A preferred lithium salt is LiPF6. The concentration of the lithium salt in the electrolyte may be in the range from ≥0.5 M to ≤2.5 M, preferably in the range from ≥0.8 M to ≤1.5 M, more particularly in the range from ≥0.9 M to ≤1.5 M. The electrolyte is prepared by introducing the lithium salt and the additive into the solvent.
Another aspect of the invention relates to an electrode, more particularly a cathode, for an alkali metal-based, more particularly lithium-based, energy storage system, including a support bearing at least one applied or deposited active material, the active material bearing an applied metal selected from Mg, Al, Cu and/or Cr and/or the active material of the composite electrode, more particularly composite cathode, being partially replaced by a metal selected from Mg, Al, Cu and/or Cr in the form of a metal powder or a metal salt. In one aspect, the metal may have been applied to the active material by sputtering.
As used herein, the term “cathode” refers to an electrode which accepts electrons on connection to a consumer. The cathode in this case, is also referred to as the “positive electrode”. As used herein, the term “active material” is a material which is able to accept and give up lithium ions reversibly, a process referred to as “insertion”. The active material therefore participates “actively” in the electrochemical reactions which take place during charging and discharging, in contrast to other possible constituents of an electrode such as binder, conductive carbon, or support. The active material is customarily applied to a metal foil support, as for example a copper or aluminum foil, or to a carbon-based current collector foil, functioning as a current collector.
The application, more particularly the sputter deposition, of a metal selected from Mg, Al, Cu and/or Cr onto the cathode may advantageously also improve the compatibility of electrode and electrolyte in the case of increased end-of-charge potentials. Without being tied to any particular theory, it is assumed that the sputter application of the metal likewise results in formation on the cathode of a passivating layer which protects the electrode and the electrolyte from decomposition.
It is understood that the layer of metal applied to the cathode by sputtering or otherwise is not continuous, so that the charging and discharging operations are unaffected. The surface of the cathode may be covered partially with a metal selected from Mg, Al, Cu and/or Cr, in the form, for example, of thin-film metal dots. Dot structures or similar structures of this kind can be applied to the active material by sputtering, using corresponding masks. The diameter of the metal dots may be in the range ≤12 mm, preferably in the range from ≥1 m to ≤6 mm, more particularly in the range from ≥0.1 mm to ≤5 mm, as an example at 3 mm, 2 mm or 1 mm. The layer thickness of the metal dots may be in the range of ≤5 m, preferably in the range from ≥50 nm to ≤5000 nm, more particularly in the range from ≥500 nm to ≤1000 nm. The sputter-applied metal dots in particular do not influence the normal charging and discharging operations. This has the advantage that there is no resultant adverse effect on normal battery operations.
Advantageously, the partial replacement of the cathode active material by a metal selected from Mg, Al, Cu and/or Cr in the form of a metal powder or a metal salt, more particularly by partial replacement of the active material in the suspension of the composite electrode, more particularly composite cathode, during the production of the composite electrode, more particularly composite cathode, may also reduce the capacity attenuation of the lithium energy storage system, may reduce self-discharge, and may reduce the overall internal resistance of the lithium energy storage system. In contrast to EP 1 056 143, however, the suspension is not heated to a temperature above the decomposition temperature of the metal powder or metal salt. Instead there is only drying under reduced pressure, such as oil-pump vacuum, for example. Additionally, preferably, the suspension is heated. For this purpose, it is possible to use a temperature of 30 to 330° C., more preferably 80 to 290° C., especially preferably 100 to 150° C. In this way there is no decomposition or conversion of the metal powder or metal salt into magnesium oxide. The metal powder or the metal salt is therefore still in the form of a metal powder or metal salt in the completed composite electrode, alongside the active material. The metal, preferably magnesium, may be present in an amount of 0.1 to 10 wt %, preferably 0.5 to 5 wt %, especially preferably 1 to 3 wt %, based on the cathode active material in the cathode.
The active material may be selected from lithium or from lithium metal oxides or lithium metal phosphates such as LiCoO2 (LCO), LiNiO2, LiNiCoO2, LiNiCoAlO2 (NCA), LiNiCoMnO2, LiMn2O4 spinel, LiFePO4 (LFP), LiMnPO4, LiCoPO4, or LiNiPO4. In preferred embodiments, the active material is selected from lithium nickel manganese cobalt mixed oxide (NMC), lithium nickel manganese oxide (LNMO) and/or lithium-rich transition-metal oxides of type (Li2MnO3)x(LiMO2)1-x (Li-rich layered transition metal oxides of the (Li2MnO3)x(LiMO2)1-x type). These compounds provide a cathode active material which is stable with respect to high voltage. Lithium nickel manganese cobalt mixed oxide (NMC) is a highly promising material for 4 V batteries, lithium nickel manganese oxide (LNMO) for 5 V batteries. Lithium nickel manganese oxide (LNMO) is present in spinel structure.
Counterelectrodes used in lithium-based energy storage systems may be anodes based on materials such as graphite, lithium, silicon, tin, or lithium titanate.
A further aspect the invention relates to a method for increasing the cycling stability of an alkali metal-based, more particularly lithium-based, energy storage system having a composite electrode, more particularly a composite cathode, having an active material, and an electrolyte containing a lithium salt dissolved in solution with an aprotic organic solvent, an ionic liquid and/or a polymer matrix. The electrolyte is admixed with an additive selected from a cation or a compound of a metal selected from Mg, Al, Cu and/or Cr, and/or the metal selected from Mg, Al, Cu and/or Cr is applied to the active material of the cathode. Examples of the active material include lithium nickel manganese cobalt mixed oxide (NMC), lithium metal manganese oxide (LNMO) and/or lithium-rich transition-metal oxides of type (Li2MnO3)x(LiMO2)1-x. In another aspect, the active material of the composite electrode, more particularly composite cathode, is partially replaced by a metal selected from Mg, Al, Cu and/or Cr in the form of a metal powder or a metal salt.
The additive selected from a cation or a compound of a metal selected from Mg, Al, Cu and/or Cr may be added in the form of salt like, inorganic or organic compounds to the electrolyte, preferably with a counterion which also serves as anion of a conductive salt. The metal selected from Mg, Al, Cu and/or Cr is present preferably as a cation in conjunction with an anion selected from AsF6−, ClO4−, SbFb−, PtCl6−, AlCl4−, GaCl4−, SCN−, AlO4−, CF3CF2SO3−, (CF3)SO3−, C(SO2CF3)3−, PF6−, PF3(CF3)3− (FAP), PF4(C2O4)−, BF4−, B(C2O4)2−, BF2(C2O4)−, B(C2O4)(C3O4)−, (C2F5BF3)−, (FAB), B12F122−, N(SO2CF3)2−, N(FSO2)2− and/or N(SO2C2F5)2−. In particular, Mg, Al, Cu, and Cr may be added as a mixture of a lithium-based conductive salt with a magnesium-, aluminum-copper- or chromium-based conductive salt. The anion is preferably selected from bis(trifluoromethanesulfonyl)imide (TFSI), bis(fluorosulfonyl)imide (FSI), hexafluorophosphate, hexafluoroarsenate, perchlorate, tetrafluoroborate, difluoromono(oxalato)borate, bioxalatoborate and/or trifluoromethanesulfonate (Tf). In particular, Mg, Al, Cu, and Cr may be added in the form of a sulfonylimide salt or a sulfonate salt. Preferred salts are magnesium (II) bis(trifluoromethanesulfonyl)imide (MgTFSI2) and aluminum (III) trifluoromethanesulfonate (AlTf3). The additive may be dissolved like a lithium salt in an aprotic organic solvent, an ionic liquid and/or a polymer matrix. The magnesium, aluminum, copper and/or chromium salt may be added in a concentration in the range from ≥0.1 ppm to ≤10 wt %, preferably in the range from ≥0.01 wt % to ≤5 wt %, more preferably in the range from ≥0.1 wt % to ≤2 wt %, as an example, of 0.7 wt % or 1 wt %, based on a total electrolyte weight.
In another aspect, a metal selected from Mg, Al, Cu and/or Cr may be applied onto the active material of the cathode. The active material can be selected from lithium nickel manganese cobalt mixed oxide (NMC), lithium nickel manganese oxide (LNMO) and/or lithium-rich transition-metal oxides of type (Li2MnO3)x(LiMO2)1-x. In another aspect, the active material of the composite electrode, more particularly composite cathode, may be partially replaced by a metal selected from Mg, Al, Cu and/or Cr in the form of a metal powder or a metal salt.
In one preferred embodiment, the metal selected from Mg, Al, Cu and/or Cr is applied by sputtering onto the active material of the cathode. It is understood that the layer of metal applied by sputtering or otherwise is not continuous, so as not to impact adversely on normal charging and discharging events. The sputter application of the metal takes place preferably in the form of thin-layer metal dots. In order to produce a dot pattern of this kind, the NCM coated substrate may be provided with a correspondingly perforated mask. The diameter of the metal dots may be 3 mm, 2 mm or 1 mm. The layer thickness of metal dots may be in the range of ≤5 m, preferably in the range from ≥50 nm to ≤5000 nm, more particularly in the range from ≥500 nm to ≤1000 nm.
In one preferred embodiment, the active material of the composite electrode, more particularly composite cathode, may be partially replaced by a metal selected from Mg, Al, Cu and/or Cr in the form of a metal powder or a metal salt, by adding the metal powder or metal salt, preferably magnesium powder or magnesium salt, such as magnesium sulfate, to the active-material suspension instead of the active material, during the production of the cathode from a current collector and an active-material suspension. The metal powder or metal salt is added to the active material suspension in an amount of 0.1 to 10 wt %, preferably 0.5 to 5 wt %, especially preferably 1 to 3 wt %.
Examples of the alkali metal-based energy storage system include sodium battery, more particularly sodium air battery or sodium sulfur battery, lithium battery, lithium-ion battery, lithium-ion accumulator, lithium polymer battery and/or lithium-ion capacitor. As used herein, the term “energy storage system” embraces primary and secondary electrochemical energy storage apparatus, namely batteries (primary storage systems) and accumulators (secondary storage systems). In common linguistic usage, accumulators are frequently referred to using the term “battery”, which is widely used as a generic term. Accordingly, the term “lithium-ion battery” is used synonymously with “lithium-ion accumulator”. Presently, therefore, the term “lithium-ion battery” may likewise identify a “lithium-ion accumulator”. The energy storage system is preferably a lithium-ion battery or lithium-ion accumulator.
Another aspect of the invention relates to the use of a metal selected from Mg, Al, Cu and/or Cr for increasing the cycling stability of an alkali metal-based, more particularly lithium-based, energy storage system, having a cathode, an anode, and an electrolyte containing a lithium salt dissolved in solution with an aprotic organic solvent, an ionic liquid and/or a polymer matrix. A cation or a compound of a metal selected from Mg, Al, Cu and/or Cr is used as an additive in the electrolyte, and/or the metal selected from Mg, Al, Cu and/or Cr is applied to the active material of the cathode. Examples of the active material include lithium nickel manganese cobalt mixed oxide (NMC), lithium nickel manganese oxide (LNMO) and/or lithium-rich transition-metal oxides of type (Li2MnO3)x(LiMO2)1-x, and/or the active material in the composite electrode is partially replaced by a metal selected from Mg, Al, Cu and/or Cr in the form of a metal powder or a metal salt.
Alkali metal-based, more particularly lithium-based, energy storage systems in which Mg, Al, Cu and/or Cr are used as an additive in the electrolyte and/or as a sputter deposition, applied by sputtering, onto the active material and/or as a partial replacement of the active material in the composite electrode are suitable for all areas of application. Using, in particular, an active material of the cathode that is selected from lithium nickel manganese cobalt mixed oxide (NMC), lithium nickel manganese oxide (LNMO), and lithium-rich transition-metal oxides of type (Li2MnO3)x(LiMO2)1-x, the energy storage systems are also suitable for high-voltage applications.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.